Google appears to be testing partial payments with Google Play credits on paid purchases
According to one Reddit user, Google is now seemingly beginning to allow users to part-pay for items using their Google Play Store credit. Previously, if you didn’t have enough credit to cover the entire cost of a download, you couldn’t use it. It’s particularly frustrating if an app or download is only a few cents more than what’s in your balance. If you don’t have enough, it usually defaults to your registered credit card to pay the full amount, leaving your Google Play balance untouched.